The present invention relates generally to the field of acoustic sensing, and more particularly to acoustic sensing by means of fiber optic interferometry.
Fiber optic interferometer acoustic sensors typically measure the acoustic signal in the presence of noise by the estimation of phase information produced in the interferometer. However, such estimation is difficult, especially in water, because the phase noise resulting from the thermal fluctuations experienced by the fiber is much greater than the phase shift which constitutes the acoustic signal. These thermal phase fluctuations are so large that they cause the acoustic signal (as seen from the interference signal from the interferometer) to fade in and out. Additionally, phase fluctuations arise due to laser cavity fluctuations and signal fading due to polarization rotation of the guided light. There are two general demodulation techniques used to eliminate this fading phenomenon and recover the acoustic signal from an optical fiber interferometer.
The first technique, referred to as the heterodyne-FM method, incorporates a Bragg cell in the reference leg of the interferometer to frequency shift the light. The light is shifted in frequency by an amount equal to the frequency of the driver of the Bragg cell (typically in tens of megahertz). The interference signal of the interferometer (that results from the Bragg cell in the reference leg) is a signal that has a carrier frequency equal to that of the driver of the Bragg cell. The acoustic signal will be seen as sidebands to the carrier frequency. Heterodyning simply shifts the signal (acoustic information and thermal noise) up in frequency so that it may be demodulated. The type of demodulation employed is an FM discriminator; i.e., the system is designed to demodulate a frequency-modulated signal. This type of detection scheme will effectively demodulate the acoustic signal but it lacks utility for a practical system to be deployed in an underwater acoustic environment in the following respects:
1. The Bragg cell used in this technique requires precise alignment to shift the optical frequency. This alignment requirement will present difficulties in system layout and implementation.
2. For FM demodulation, the oscillator of the Bragg cell must be extremely stable (to about 1 part in 10.sup.7) in frequency. Such stability requires sophistication and expense in the design of the oscillator.
3. The signal is phase modulated, but it is FM demodulated. A signal at 1000 Hz will be 100 times larger than one at 10 Hz. Accordingly, the system will demonstrate a sensitivity as a function of frequency and will be increasingly limited with decreasing frequency. Thus, FM detection for a PM signal is far from an optimum approach.
The second demodulation technique, referred to as the Homodyne phase-lock detection method, correctly demodulates the phase-modulated signal generated by the acoustic field; that is, it PM demodulates the signal. The demodulation of the signal is accomplished by general phase-lock techniques that use an optical detector as the phase comparator. The output of the detector is amplified (dc coupled), low pass filtered, and then sent to a device that stretches the reference fiber (the phase-lock feedback operation that keeps the interference beams in quadrature). In general, this type of system suffers many drawbacks, all in part due to the optical detector being used as the phase comparator. In this regard the optical detector is typically a square-law device and produces a dc component that is at least, if not more than, the same level as the ac (information) signal, regardless of whether the system is in quadrature or not. The phase-locked homodyne system is a dc-coupled system. For such a system to be effective, the loop gain must be substantial in order to minimize the phase error as necessary to enhance the accuracy of phase tracking. The dc component will also be amplified in this process. Thus, if the amplifier is not compensated with a dc offset at the input, it will saturate and will not be able to track the phase. This type of offset is easy to implement if the dc level at the output of the optical detector is constant. However, in a fiber optic system the optical detector output is subject to various fluctuation in the dc level. It should also be noted that any change in the dc level in a homodyne system also changes the ac level; although it is the change in the dc level that will act to throw the system off lock and cause the amplifier to saturate, resulting in the loss of signal.
For either heterodyne or homodyne systems, fluctuations in the dc level at the detector output which hinder the system operation may arise from the following:
1. Mechanical alignment problems in the optical components in the configuration. The single-mode fibers used in these configurations have a very small diameter (typically 5 .mu.m), and any slight alteration of the alignment will result in an intensity fluctuation seen at the detector.
2. The intensity output of the source (laser) fluctuates.
3. Polarization rotation may occur in either of the legs of the interferometer. Another problem with the homodyne system is that optical detectors and sources exhibit 1/f noise. The detector output is a signal centered around 0 Hz; or, in other words, the carrier frequency of the phase-modulated signal is 0 Hz. This is the noisiest place in the spectrum to measure signals with the optical detector.
From the above, it can be seen that there are a variety of problems in current methods contemplated for measuring the acoustic signal with a fiber optic interferometer.